The system catalogs are the place where a relational database
management system stores schema metadata, such as information
about tables and columns, and internal bookkeeping information.
PostgreSQL's system catalogs are
regular tables. You can drop and recreate the tables, add
columns, insert and update values, and severely mess up your
system that way. Normally, one should not change the system
catalogs by hand, there are always SQL commands to do that. (For
example, CREATE DATABASE inserts a row
into the pg_database catalog — and
actually creates the database on disk.) There are some exceptions
for particularly esoteric operations, such as adding index access
methods.